This invention relates to a method of and means for controlling the height of a plasma arc torch above a workpiece and more particularly to such a method and means specifically designed to automatically maintain a selected height of the plasma arc torch above the workpiece during actual operation by sensing conditions present within the arc itself to effect the desired control.
A plasma cutting torch makes use of a high voltage, high current, DC arc that is established between the torch head and the working surface in a stream of gas thereby ionizing the gas and producing the desired plasma. Initial ionization of the gas is most conveniently accomplished by the application of a short duration, high frequency, high voltage pulse across a portion of the path of the gas flow within the torch head. After such pulse has established a high frequency arc within the torch head the flow of gas and the DC potential between the torch head and the workpiece will cause the arc column to stretch and extend beyond the nozzle so that when the torch is properly positioned with respect to the surface of the workpiece the arc transfers to the work surface and is maintained by the high DC potential between the torch head and the workpiece. The high frequency, high voltage source is, of course, disconnected from the torch as soon as the DC arc has been established between the torch and the workpiece. Although other means may be employed for initial ionization of the gas flow between the torch and the workpiece, the above described transferred arc system is typical of most commercially available plasma arc cutting torches.
The quality of the cut produced by a plasma arc torch is primarily dependent upon three variables:
1. DC power (voltage and current);
2. GAS PRESSURE;
3. TORCH TO WORK DISTANCE.
The first two variables are reasonably well controlled in most commercial plasma arc cutting torch systems. However, torch to workpiece distance is difficult to maintain when cutting workpieces having warped or uneven surfaces. It is important that this variable also be closely controlled because proper torch to work spacing not only permits cleaner and more accurate cuts but also allows higher cutting speeds and reduces the objectionable noise attendant to all plasma arc cutting processes.
Most commercial plasma arc cutting torch systems are provided with torch height controls as part of the standard equipment. In the prior art these controls have employed capacitive, eddy current, pneumatic, etc. sensors. These types of controls and sensors are reliable as positioning means in the absence of any external disturbing factors and are useful in plasma arc systems in establishing approximate torch to workpiece spacing prior to initiation of the cutting operation. However, once the cutting operation is initiated by establishment of the plasma arc, conditions arise which render such controls and sensors unacceptable as a means to maintain the proper distance between the torch and surface of the workpiece. This is particularly true in systems using a water stabilized arc and a water muffler system since various highly turbulent electrical and mechanical effects are set up in the area between the torch and surface of the workpiece (as well as on the surface), not only by the plasma arc itself but also by the water system which will seriously degrade the function of height controls employing the above mentioned types of sensors.
It will be understood that the above mentioned types of sensors must be located in close spaced relation to the plasma arc torch above the surface of the workpiece and that they are inherently expensive in order to be sensitive enough for effective operation. Thus, not only must they be located in a position where they will be most subject to the turbulence effects mentioned above, but they will also be exposed to damage in the harsh shop environment in which plasma arc cutting systems are used.
It is an object of this invention to provide a method of and means for automatically maintaining a selected height of a plasma arc torch above the surface of a workpiece which will not be affected by external disturbances or turbulence effects present at and about the plasma arc cutting torch in operation.
It is a further object of this invention to provide a method of and means for automatically maintaining a selected height of a plasma arc cutting torch above the surface of a workpiece without the necessity of employing sensors physically located in close proximity to the plasma arc torch.
According to the method and means of this invention information is obtained from the plasma arc torch power supply that is directly related to conditions within the plasma arc itself. Such information is utilized to effect the desired control of the height of the plasma arc torch above the surface of the workpiece. Thus the method of this invention avoids the effects of external disturbances and turbulence present at and about the plasma arc torch which do not directly affect the plasma arc itself and the means may be physically located in close proximity to the power supply for the plasma arc torch where it can be protected from physical damage and provided with appropriate isolation means to protect it from transient electrical effects and other disturbances and turbulences.